The depth of the pond in a decanter centrifuge is particularly relevant to its successful operation. This fact is particularly true when the centrifuge is operating in an "above spillover" condition. An above spillover condition occurs when the pond surface in the bowl of the centrifuge is radially inward of the solids weir surface.
The operational characteristics of a decanter centrifuge in an above spillover condition are described in various forms in Ambler U.S. Pat. No. 3,172,851 and Lee U.S. Pat. No. 3,795,361. In the Ambler device, a solids dam must form at the solids discharge weir in order for the liquid layer to provide its contemplated hydraulic assistance to the solids discharge. In the Lee construction, the baffle projecting from the conveyor hub must penetrate and seal with the solids layer within the centrifuge bowl to create a centrifugal pressure head assist to the conveyor in the discharge of the separated solids. The relative difference in radius between the solids and liquid weir surfaces can be used to create the above spillover condition and be used to control the operation and performance of a decanter centrifuge. However, precise control over the parameters of the above spillover operation is often required. (The Ambler and the Lee patents as identified above are herein incorporated by reference.)
During start up of a decanter centrifuge operating in an above spillover condition, a solids layer must be built up within the bowl in order for the centrifuge to reach a steady state operation. In both the Ambler and Lee type operation, because the solids weir is typically radially outward of the liquid weir, the liquid feed mixture will discharge over the solids weir during start up prior to reaching steady state operation. This condition will also occur during a "wash-out", until the dam or seal are again formed. The reformation of the dam or seal to again achieve a steady state operation may require significant operator attention and result in substantial loss of operation time for the centrifuge. Liquid discharge through the solids discharge ports is normally considered unacceptable.
LaMontagne U.S. Pat. No. 4,575,370 shows a liquid discharge weir for a decanter centrifuge that operates under different conditions based upon the feed rate into the centrifuge. In LaMontagne, the weir plates for the liquid discharge include a notch or the like which interrupts the weir surface. At low flow rates, the liquid is discharged through the notch which is at a position radially outward of the solids discharge weir. At higher flow rates, such as that of normal operation, the flow over the liquid discharge weir is great enough to raise the level of the pond within the bowl radially inward of the solids discharge weir, i.e., to an above spillover condition. Thus, LaMontagne contemplates that the flow rate may be used to control the operational characteristics of start up and prevent liquid discharge through the solids discharge ports. (The LaMontagne patent identified above is herein incorporated by reference.)
The Lee decanter centrifuge creates a centrifugal pressure head which directs the solids through the annular passageway defined by the baffle periphery and the bowl wall to assist in the discharge of the solids. It has been found that this pressure head substantially improves the operation of a decanter centrifuge in particular with respect to a thickening type operation. The Lee type centrifuge has also been found applicable to what is known as "difficult-to-convey" type solids. These difficult-to-convey solids are normally not dischargeable from a decanter centrifuge by the screw conveyor alone and often require the use of polymers or the like to create acceptable separation.
In certain situations in the operation of a Lee type decanter centrifuge, the concentration of the solids discharge is difficult to control. In waste water thickening, the production of a cake having up to a 4% solids concentration is possible typically on a consistent basis. Also, the production of a solids concentration in excess of 8% is consistently possible. However, in the range between 4% and 8%, it is often difficult to produce a consistent cake concentration. The reason for this difficulty is attributed to the inability to precisely control the pond level and to adjust for changes in feed rate and feed solids concentration.
Operation of a Lee type decanter centrifuge in an above spillover condition has been found to be most advantageous in waste water thickening. However, similar advantages have been found for an above spillover condition in the concentration of solids in a dewatering-type operation, in which the solids concentration is usually greater than 20%.
It is known to use an inflatable type dam within a centrifuge for creating a liquid-liquid separation. Such an inflatable dam is shown in Sharples U.S. Pat. No. 3,179,334. However, a liquid-liquid type separation includes different operational characteristics and parameters from those in a liquid-solids type separation of a decanter centrifuge. In the operation of a decanter type centrifuge, the depth of the pond within the centrifuge is often critical to successful operation and in controlling the characteristics of the separated solids discharge and the liquid centrate. Variation in the depth of the pond in a liquid-liquid separation is useful in locating the liquid-liquid interface to control liquid clarity.
The relative change in operation of a centrifuge used for a liquid-liquid type separation as compared to that of a decanter centrifuge used for a liquid solids type separation as a function of the radial difference between the weir surfaces of the separate discharges displays the differences in operational characteristics and the different parameters of operation of the two types of centrifuges. The following is an outline of some of these differences, making reference to a Lee type decanter centrifuge.
One operational characteristic of a centrifuge is the location in the centrifuge bowl of the interface between the lighter density material (typically a liquid) and the heavier density material (either a liquid or a solids type material). In a liquid-liquid type separation, the interface is considered to be relatively sharp and well defined. However, in a decanter type centrifuge this sharp definition is not necessarily found. The location of the interface in a liquid-liquid centrifuge is the function of the density ratio of the two liquids being separated and the relative distance from the axis of rotation of the weir surfaces for the two materials being discharged. The location of the interface in a Lee-type decanter centrifuge is a function of the density difference between the liquid and solids (with the solids concentration varying at different radial positions in the bowl), the relative radial difference between the weir surfaces, the rate of the feed into the bowl, the solids concentration in the feed, the differential speed of the screw conveyor with respect to the bowl, the speed of rotation of the bowl, and the compactability of the solids material.
A change in the feed rate in a decanter centrifuge will result in a change in the location of this interface and the discharge characteristics of the solids. An increase in the feed rate will typically result in the interface moving radially inward. Feed rate changes in a liquid-liquid type separation do not result in substantial changes in the discharge characteristics or a movement of the interface.
Moving the light liquid discharge weir surface radially inward in a liquid-liquid separation (i.e. raising the pond surface with respect to the heavy liquid discharge weir) will result in the interface moving radially outward. However, the flow rate of both the light and heavy liquids are not affected by the change in weir location. If the liquid discharge weir surface is moved radially inward in a Lee-type decanter centrifuge, the interface will move radially outward and the solids discharge flow rate will increase as the cake concentration in the solids decreases. This change is a function of the centrifugal pressure head being increased by the higher level of liquid above the solids discharge weir. This increased pressure head results in greater assist to the conveyor in discharging the solids and, thus, the faster solids discharge flow rate. The solids concentration in the discharge will also decrease because, at a constant rate of solids in the feed and a greater output flow rate, there is a net decrease in the amount of solids retained in the centrifuge bowl. The interface is moved as a result of the increase in the centrifugal pressure head in the separation zone and the reduction in solids concentration.
These operational differences between a decanter centrifuge and a liquid-liquid type separation can be attributed to the fact that the two centrifuges are structurally different and function according to different physical principles. A liquid-liquid type separation does not provide a supplemental discharging force as a result of the variation of the relative position of light liquid discharge weir as in the Lee type decanter centrifuge. Manifestly, any similarities between the presently contemplated structure and prior structures are not suggestive of the present invention.